In a hybrid drive vehicle in which an electric motor and an internal combustion engine are connected by a clutch and power of the internal combustion engine is used in addition to that of the electric motor to drive the vehicle in response to a torque request, two types of travel modes are, for example, selectively applied. Specifically, there are an EV travel mode for travel using only power of the electric motor and an HEV mode for travel using power of both the electric motor and the internal combustion engine.
The EV mode and the HEV mode are determined, for example, based on a depression amount of an accelerator pedal. Specifically, the vehicle travels in the EV mode if the depression amount of the accelerator pedal is not higher than a threshold value while traveling in the HEV mode if the depression amount of the accelerator pedal is higher than the threshold value. In switching from EV mode traveling to HEV mode traveling, the clutch is connected and the electric motor activates the internal combustion engine by cranking the internal combustion engine. Disengaging the clutch makes a switch from the HEV mode to the EV mode.
If the operation of the internal combustion engine is stopped in the switch from the HEV mode to the EV mode, the start and stop of the internal combustion engine are frequently made when the depression amount of the accelerator pedal varies near the threshold value and a driver may feel uncomfortable.
JP2010-143423A, published in 2010 by the Japan Patent Office, proposes a control to alleviate such a hunting phenomenon. According to this prior art, a start or stop of an internal combustion engine is prevented if a depression amount of an accelerator pedal changes in an opposite direction and reaches a threshold value immediately after the start or stop of the internal combustion engine is requested.